The re-sharpening of twist drills presents many difficulties both for professional engineer and amateur alike. The main problem lies in the complex shape of the twist drill tip which is rarely fully understood.
Although a twist drill appears at a casual glance to be no more than a rod-like body with helical flutes, the precise configuration is in fact much more complex than this; the twist drill tip, in particular, has a shape which can only be appreciated by careful study. For example, the "pointed" end of the tip is in fact a chisel edge and the generally conical "lands" on either side of this are neither flat nor cylindrically curved but should have a curving inclination from the cutting edge to the trailing edge with respect to a plane perpendicular to the drill axis and a curvature which varies radially of the tip to approximate a conical surface.
Various drill sharpening tools are known in the prior art. In one such known tool there are provided guides operable to grind "flats" on the end faces of the drill by running the drill along a flat grindstone; this is largely unsatisfactory, however, since the tip end surfaces should be curved. Another known device acts to hold the drill in a required orientation about its axis, and to sweep the tip of the drill about an axis inclined at an acute angle to the longitudinal axis of the drill whilst passing the tip end surface of the drill across a flat or curved grindstone surface. The fundamental disadvantage of this arrangement lies in the fact that the two "lands" of the drill tip surface are separated by a chisel edge and are effectively independent. By grinding the two "lands" separately the problem arises of longitudinally aligning the drill sufficiently accurately for both "lands" to be ground symmetrically with respect to the longitudinal axis of the drill. A recent example of a drill grinding tool of the former type is discussed in British Patent No. 1,468,327 granted to Robert Bosch GmbH. In the Bosch drill grinding tool a cylindrical grindstone is carried in a holder which has an extension forming a plate-like housing extension which is supported on a web. The outer surface of the housing extension forms a guide plane for a guiding device consisting of a supporting carriage which serves for guiding a drill to be ground during the grinding procedure. The supporting carriage has a slide block which can be longitudinally guided in a guide machined in the plate-like housing extension, this guide being made in the form of a straight slot in the housing extension extending parallel to the axis of the grinding spindle. The supporting carriage carries a rectilinear V-section groove in which the drill to be ground is inserted during the grinding procedure, the drill being held fast by hand and pressed against the curved surface of the grinding wheel whilst the carriage is moved along the recrtilinear guide. This grinds a concave cylindrical surface on the drill tip on one side of the chisel edge and the drill must be repositioned to drill the other land. It is apparent that the concavely curved land surface formed by this apparatus in no way resembles the correct shape for a drill tip, and it is clear that the Bosch drill grinding jig can only be used for drills having a hard metal insert such as are used for drilling masonry, the insert being subsequently bevelled after the grinding operation discussed above.
A later attempt to produce a drill grinding jig capable of forming a more accurate drill tip surface resulted in the grant of British Patent No. 1,526,169 to Robert Wolff. In Mr. Wolff's drill grinding jig a stand is clamped onto a rest which has two hooks which enclose a grinding wheel. A twist drill to be sharpened is carried in a channel in a holder which is pivoted to a support clamped to the housing with respect to which the grindstone turns. The pivot axis about which the holder turns may be parallel to the grindstone axis or inclined thereto at a predetermined angle, and the pivot axis about which the drill is turned is offset from the axis of the drill and perpendicular thereto so that as the carriage is turned about its pivot axis the tip of the drill is swept across the curved surface of the grindstone. Once one land has been ground in this way it is necessary to remove the drill from the holder, reposition it with the other cutting edge horizontal and perform a new sweep by swinging the drill holder about its pivot axis.
Because the two lands are ground independently it is not possible accurately to set the drill so that both cutting edges will be exactly symmetrical. It will be appreciated that since the flutes of a twist drill are helical, any removal of material from the drill tip will result in the land being shifted angularly as well as axially so that the original setting of the cutting edge parallel to the grindstone axis (or horizontally assuming that the grindstone runs on a horizontal axis) will result in a cutting edge which is inclined to the horizontal to a greater angle the more material is removed from the land during sharpening. Unless exactly the same amount of material is removed from each land then the offset of the cutting edge from its intended position will be different for each land. Further, the Wolff device does not act to cut the correct pseudo-conical surface to the drill tip, but cuts a purely cylindrical surface having a radius of curvature determined by the offset of the pivot axis of the drill holder from the drill tip itself, and this means that the variation in the "fall" or clearance between the leading and trailing edges of the cutting land is not obtained.